But do I understand the thinking behind extortion/blackmail law?  Neeeiiiggghhh.

From what I can gather having casually observed this case, in which a CBS news producer sought the American dream of making lots of money without doing lots of work, specifically by asking Dave Letterman to pay him $2 million not to go public with details of Dave's extramarital affairs with interns, the producer, Robert Joel Halderman, threatened to

• say something
• which was true
• for which he had proof

but would agree not to say it if Dave paid him.  This seems like it could be a good deal for somebody like Dave, under the right circumstances.  If it's worth $2 million to him for the true, verifiable information not to be made public, he wins, and certainly Halderman wins. 

But, you say, nothing prevents Halderman from coming back later and asking for more.  That's true, but is only true because what he did was against the law, and I don't understand why that is, so that's a bit circular.  If it weren't against the law, Dave and Halderman could enter into a legally binding agreement with non-disclosure provisions and heavy penalties if Halderman reneged on them.  Alas, promising not to say something true that you can prove if satisfied financially is against the law, so such a contract wouldn't be enforceable.  I really have never gotten it, this extortion/blackmail business.